Olefin polymerizations are frequently carried out using monomer, diluent and catalyst and optionally co-monomers and hydrogen in a reactor. When the polymerization is performed under slurry conditions, the product consists usually of solid particles and is in suspension in a diluent. The slurry contents of the reactor are circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. The product is discharged by means of settling legs, which operate on a batch principle to recover the product. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry.
Alternatively, the product slurry may be fed to a second reactor serially connected to the first reactor where a second polymer fraction may be produced. Typically, when two reactors in series are employed in this manner, the resultant polymer product is a bimodal polymer product, which comprises a first polymer fraction produced in the first reactor and a second polymer fraction produced in the second reactor, and has a bimodal molecular weight distribution. The resultant product will also usually consist of solid particles in suspension in a diluent and will then be discharged from the second reactor using settling legs in a similar way as explained above.
The product slurry recovered in an olefin polymerization process comprises a slurry of polymer solids in a liquid that contains diluent, dissolved unreacted monomer, and optionally dissolved unreacted co-monomer. Typically this liquid also includes traces of heavier elements, e.g. oligomers, and lighter components including H2, N2, O2, CO and/or CO2. Catalyst will generally be contained in the polymer.
Once recovered from the reactor, the product slurry is discharged to a flash tank, through flash lines, where most of the diluent and unreacted monomers and optionally unreacted co-monomers are flashed off. Afterwards, it is highly desirable to further treat the vapors in order to recover the unreacted monomer, optionally unreacted co-monomer and the diluent, since there is an economic interest in re-using these separated components including the monomer, co-monomer, and the diluent, in a polymerization process.
It is known in the art that a vaporous stream comprising unreacted monomer, unreacted co-monomer and diluent issued from the effluent of a polymerization process may be treated in a distillation system for separation of its components. Traditionally, the diluent is captured through a complicated process so that such diluent can be recycled to the reactor.
U.S. Pat. No. 4,589,957 for instance describes a separation process of a hydrocarbon-containing vaporous stream comprising monomer, co-monomer and diluent issued from the effluent of a homo-polymerization and/or co-polymerization process. The described process comprises subjecting the vaporous stream to a two-stage distillation provided with a common accumulation zone wherein the condensate from the accumulation zone serves as the source of feed for the second distillation and reflux for the first distillation.
However, a problem encountered in many distillation systems, is that there is a sub-optimal separation of lighter components, including H2, N2, O2, CO and/or CO2, from recovered diluent. As a consequence, use of separated diluent streams containing these components in a polymerization process may seriously reduce polymerization efficiency and result in sub-optimal polymerization conditions. Especially in the case of re-using separated diluent streams in a polymerization process for preparing bimodal polymer product, it is for instance required to recover diluent streams wherein the residual amount of lighter components such as hydrogen, is substantially reduced in order to be able to use these diluent streams in reactors wherein the higher molecular weight component of a bimodal polymer product is prepared.
An example of a recovery process that is currently applied to meet this requirement involves the production of large amounts of diluent streams that are substantially free of olefin. However, such recovery process involves the re-utilisation of a diluent stream which is in fact too pure for that purpose as it substantially lacks olefin monomer and hence also is too costly for that use. Moreover, separation methods adapted to recover large amounts of substantially olefin-free diluent entail a number of problems and disadvantages, including inter ails requiring high amounts of energy for carrying out the separation process; resulting in increased amounts of olefin monomers that have to be separated from lighter components such as those given above; increased loss of olefin monomer, reduced stability of distillation systems; etc.
In view of the above, there remains a great need in the art for optimised methods for recycling hydrocarbon-containing feed streams that need to be separated into streams that can be recycled to a polymerization process, especially wherein bimodal polyolefins, such as for instance bimodal polyethylene, is prepared. Furthermore, there is a need in the art to provide a diluent recycle process that is less expensive to construct and/or to operate.